This invention relates to a multidirection multiplex communication system, and more particularly to a system of improving the transmission efficiency of a multidirection multiplex communication system utilizing a demand assignment protocol.
The multidirection multiplex communication system is constructed to perform efficient communications between a central station and many subscriber stations scattered geometrically. Such system is desired to have a simple configuration and a high transmission efficiency for transmitting comparatively small information capacities.
In this system, the central station transmits information to the subscriber stations in the form of a time division multiplex (TDM) mode, and each subscriber station demultiplexes the received signal to an information addressed thereto. Each subscriber station, on the contrary, transmits information to the central station in the form of a burst signal during an assigned time duration for the subscriber station. Such burst signal is termed as a time division multiple access (TDMA) burst signal.
In this system, the central station provides the reference on the TDM/TDMA frame for all subscriber stations, and each subscriber station transmits the burst signal in accordance with the TDM/TDMA frame regenerated from the received signal sent from the central station.
In case of a pre-assigned mode, in which the position and duration of the burst signal of each subscriber station is pre-determined, transmission path correspnding to each burst is always established between the central node and the subscriber station. Therefore the central station can always detect time deviation of each burst from where it should be. By sending the detected time deviation to respective corresponding subscriber station, the transmission timing at the subscriber station can be controlled so that the received signal at the central station is regenerated at the optimum sampling position. Consequently, it is not necessary to recover the clock signals from the burst signals sent from respective subscriber stations to regenerate the data in a demodulation system at the central station. Accordingly, it is not necessary to add so-called preamble words for clock recovery to the tops of the burst signals sent to the central station from respective subscriber stations. It is also possible to eliminate preamble words for carrier recovery in PSK (Phase-shift-keying) system by adopting differencially coherent detects scheme. Thus it is possible to form a multidirectional multiplex communication system with high transmission efficiency in case of pre-assigned system.
In view of the efficient utilization of the frequency band, the pre-assigned system has a defect that the time slots assined for respective subscriber stations are always occupied ever when the subscriber stations do not need to transmit signals whereby other subscriber stations cannot utilize their time slots.
Improving this inefficiency of the pre-assigned system, a demand assigned system assignes time slots for respective subscribers only when they need to transmit signals. In this case, there exist silent subscriber stations in the system which transmit no burst to the central station. Thus, different from the pre-assigned system, it is impossible to control the transmit timings of these silent subscriber stations in the demand assigned system. Accordingly it is necessary to add preamble words for timing recovery to the heads of the burst signals sent from the subscriber station.
Thus, in the prior art multidirection multiplex communication system, with regard to the improvement of the transmission efficiency between the central and subscriber stations, the advantages of the pre-assigned and the demand assigned system contract with each other.